In power amplifiers used in base stations in mobile or wireless communications, the improvement in the power added efficiency (PAE) is required in order to reduce operational costs. One of the PAE improvement technologies that have recently attracted attention is envelope tracking (hereinafter referred to as ET). In an RF power amplifier, PAE is defined as the ratio of the difference of the output and input signal power to the DC power consumed.
Before describing ET, the characteristics of the transistor used in a common power amplifier will be described. The output power and PAE characteristics with respect to the input power of the transistor will be described with reference to FIG. 1. In FIG. 1, the horizontal axis represents the input power, and the vertical axis represents the output power and PAE. As the input power increases, the output power saturates. The PAE is highest in the vicinity of the saturated output power.
Referring to FIGS. 2A and 2B, description will be given of the output characteristics and the PAE characteristics when the transistor power supply voltage Vdd is varied by 5 V from 50 V to 10 V. In FIG. 2A, the saturated power of the transistor varies depending on the power supply voltage. In FIG. 2B, the input power value at which the PAE is maximum is different for each power supply voltage.
Next, ET will be described. ET is a technology that improves the PAE by allowing the power supply voltage of the transistor within the power amplifier to vary according to the envelope of an input signal. The principle of ET will be described with reference to FIG. 3. In FIG. 3, essential components of ET include an envelope detection unit, an envelope tracking power supply (ET power supply), and a power amplifier.
The envelop information of the input signal that is detected by the envelope detection unit is transmitted to the ET power supply. The ET power supply outputs a voltage having a shape corresponding to the envelope information. In this way, the power amplifier (transistor) allows the power supply voltage to vary to be able to typically operate with the maximum PAE in the vicinity of the saturated power, with respect to the input power. Thus, the ET improves the PAE. FIG. 3 shows an example of operating the ET by detecting the envelope from the transmitted RF signal. However, it is also possible to control the ET operation by a discrete voltage value by using the signal amplitude obtained by digital calculation.
The PAE when the power supply voltage of the transistor is fixed and the PAE when the ET is operated will be compared with reference to FIG. 4. In FIG. 4, the horizontal axis represents the input voltage, and the vertical axis represents the PAE. The PAE when the power supply voltage is fixed is lower than the PAE when the ET is operated. The ET uses the characteristics of the transistor. The transistor allows the power supply voltage to vary to be able to typically operate with the maximum PAE in the vicinity of the saturated power, with respect to the input power. Thus, the PAE when the ET is operated is higher than when the power supply voltage is fixed in a wide input power range.
Particularly, it is possible to take advantage of the characteristics of ET that the PAE is high in a wide input power range with respect to signals with the peak power higher than the average power, as in the digitally modulated waves using modulation schemes such as Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), and Frequency Division Multiplexing (FDM) that are used in recent mobile communication.
However, when the ET is applied to the signal with the peak power higher than the average power, it is necessary to have an ET power supply with a wide dynamic range to be able to properly output any voltage.
Known examples of ET power supply are disclosed in JP-A No. 511065/2008 and JP-A No. 277806/1987. The background of the technology will be described with reference to FIGS. 5A and 5B. JP-A Nos. 511065/2008 and 277806/1987 both disclose a method for configuring an ET power supply by using a plurality of voltage supplies. In FIG. 5A, an ET power supply 1300 includes a plurality of constant voltage supplies that are provided so that the rated supply voltage Vdd of a power amplifier is equally divided. The ET power supply 1300 outputs a voltage corresponding to the envelope signal from the envelope detection unit. The output voltage of the ET power supply having a plurality of voltage supplies has a step-like shape as shown in the dashed line in FIG. 5B.
JP-A No. 136309/1998 discloses a technology called Peak Factor Reduction (hereinafter referred to as PFR) that reduces the difference between the average power and the peak power of a signal by reducing the peak power included in the signal. The PFR prevents the high power input itself. The PFR reduces the output dynamic range that is required for the power supply to allow the power supply voltage to precisely vary with respect to the input range of higher occurrence probability. Thus, the PFR improves the PAE. The PFR is the technology that allows the power amplifier operating at a fixed voltage to reduce the peak power included in the signal. This can reduce the difference between the average power and the peak power of the signal, allowing the power amplifier to operate at higher PAE. This technology is currently used by many mobile communication transmitters.
JP-A No. 198513/1991 discloses a power amplifier of a combination of ET and distortion compensation. Further, JP-A No. 336626/2004 discloses a power amplifier that makes the power supply voltage constant at low output power by changing the power supply voltage at higher output power.